An efficient mesoporous carbon nitride (g-C3N4) functionalized Pd catalyst for carbon-carbon bond formation reactions

Article abstract of DOI:10.1039/c6ra04170a

Metal nanoparticles in pristine form without any stabilizing agents and free from agglomeration are very critical for their function and diverse catalytic applications. With the goal to accomplish a molecularly defined Pd-based heterogeneous C-C bond form

Full text of DOI:10.1039/c6ra04170a

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An Efficient mesoporous carbon nitrides (g-C3N4) functionalized Pd catalyst
for carbon−carbon bond formation reactions
S. Elavarasan,a B. Baskar,a C. Senthil,a Piyali Bhanja,b A. Bhaumik,b P. Selvam,♣ c and M.
a
Sasidharan ∗
aSRM Research Institute and Department of Chemistry, SRM University, Kattankulathur,
Chennai, 603203, India, Eꢀmail: sasidharan.m@res.srmuniv.ac.in
bDepartment of Material Science, Indian Association for the Cultivation of Science, Jadavpur,
Kolkata 700 032, India
cNational Center for Catalysis Research and Department of Chemistry, IIT
−Madras, Chennai,
600 036, India.
♣Adjunct Professor, New Industry Creation Hatchery Centre, Tohoku University, Sendai, Japan.
∗ To whom the correspondence should be addressed
Metal nanoparticles in pristine form without any stabilizing agents and free from agglomerations
are very critical for their function and diverse catalytic applications. With the goal to accomplish
a molecularly defined Pdꢀbased heterogeneous C−C bond forming catalysts, highly ordered
mesoporous nitrogenꢀrich carbon nitride (MCN) polymers with extended threeꢀdimensional πꢀ
conjugation have been used as solid supports. Here, palladium nanoparticles ca. Pd(II) and
Pd(0) were dispersed onto MCN and used in surfaceꢀexposed state that renders them inherently
high catalytic activity. The catalysts were thoroughly investigated by a series of characterization
13
1
techniques such as smallꢀangle XRD, TEM, EDAX, SEM, C MAS NMR, H NMR, FTIR, N2
sorption, and CHN analyses. The XRD, N2 sorption isotherms and TEM show that Pdꢀcatalysts
maintains hexagonal mesoporous structure with high surface area (355.9 m2/g) and pore volume
of 0.63 mL/g. The 13C MAS NMR and FTIR spectroscopy have confirmed the presence of
triazine ring, NH2 and NH groups in the polymeric graphitic carbon nitrides. Both Pd(II) and
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Pd(0) catalysts exhibited good catalytic activities and product selectivities in copper− and
phosphine−free Sonogashira crossꢀcoupling reaction between aryl iodide and aryl alkynes. Hot
filtration tests confirmed the heterogeneity of the catalysts and the catalyst was reused for six
successive reactions with negligible change in the conversion.
Introduction
The Sonogashira cross−coupling reaction is one of the most widely used carbon ̶ carbon bond
forming reactions in organic synthesis,1 as it has great potentials for the synthesis of fine
chemicals and therapeutic drugs and their intermediates.2−6 The Sonogashira coupling of aryl
halides with alkynes provides a versatile method to generate a wide variety of organic
compounds such as conjugated oligomers, polymers, symmetrical diynes, and materials for
nonlinear optical and molecular electronics.7−10 In a typical Sonogashira reaction, the coupling
reaction is carried out by using a homogeneous phosphineꢀligated palladium complex with a
catalytic amount of copper(I) iodide as a coꢀcatalyst in the presence of a base under an inert
atmosphere using an organic solvent.3,4 Phosphorous ligands are usually poisonous, airꢀsensitive,
unrecoverable, and can degrade at elevated temperatures.11 Copper salt is used to facilitate the
oxidative addition of acetylene to palladium metal, but it generates dangerous waste materials
that are difficult to separate.12 Furthermore, a problem that accompanies soluble Pd complexes is
the loss of the expensive metals. Consequently, most of the traditional methods were
economically and environmentally unviable and modifications were carried out by introducing
phosphine−, copper−, and amine−free catalytic systems.13,14
Although, homogeneous transition metal catalysts remain the most active catalysts for
Sonogashira reactions, it suffers from numerous drawbacks like high cost of ligands, the catalyst
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cannot be regenerated/reused, and difficult to separate environmentally polluting heavyꢀmetal
ions from reaction mixture12 Moreover, the homogeneous palladium catalysts tend to lose their
catalytic activity because of palladium metal aggregation and precipitation. Heterogenization of
the existing homogeneous palladium catalysts could be an attractive solution to circumvent these
problems. The design of heterogeneous transition metal catalysts has to take into account several
basic characteristics including high efficiencies and selectivities as well as environmental
consideration. In this context, zeoliteꢀsupported reaction system,15 MCMꢀ41ꢀPd(0),16 Pd(0)ꢀ
Montmorillonite,17 PdꢀPPy,18 Pd(OAc)2/Agarose,19 polystyreneꢀPd(II),20 and SiꢀPVIꢀPd,21 were
investigated under heterogeneous reaction conditions. Although these methods are valuable,
many of these methods have significant drawbacks such as the tedious work up procedures, use
of phosphine and copper salts, high reaction temperatures and low yields. Alternatively, porous
organic polymers, a class of highly crossꢀlinked amorphous polymers possessing nanopores have
been used as support to obtain Pdꢀcatalyst (Pd/COFꢀLZUꢀ1) for Suzuki coupling reactions.22
Pitchumani et al. have reported a microporous networked polymer as support to prepare Pdꢀ
catalyst for Sonogashira coupling reactions.23 Recently, we also reported a triazine
functionalized polymer as a novel support for Pdꢀmediated C−C crossꢀcoupling reactions.24 The
Pdꢀcross coupling reactions has also been explored using Chitosan Microspheres and dualꢀ
module microporous mesoporous polymers as supports.25,26
Recently organic polymeric semiconductors, namely graphitic carbonꢀnitrides with
general formula C3N4 with extreme chemical and thermal stabilities have been investigated as
heterogeneous catalysts for diverse chemical transformations.27−29 For instance, it has been
investigated for selective oxidation of alcohols and benzene,30,31 hydrogen evolution,32
hydrogenation,33 FriedelꢀCrafts reaction,34 CO2 reduction,35 and visible light photocatalysis.36
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The unique features of C3N4 are that abundant nitrogen functionalities on the surface sites acts as
strong Lewis base sites, while the π−bonded planar layered configurations are utilized to anchor
desired metal which allows its direct use as heterogeneous catalysts. However, to the best of our
knowledge, polymeric graphitic C3N4 has not been investigated for C−C bond formation
reactions through crossꢀcoupling reactions. Our strategy for developing a heterogeneous catalyst
for Sonogashira reaction is based upon the concept of (i) immobilizing a molecularly dispersed
Pd(II) complex with nitrogen atoms of mesoporous C3N4(Pd(II)ꢀC3N4) and (ii) spatial
confinement and electronic stabilization of Pd(0) nanoparticles within a mesoporous C3N4
π−network (Pd(0)ꢀ C3N4) in which surfaceꢀexposed nanoparticles are crystallized upon in inꢀsitu
reduction. In the above catalysts, Pd nanoparticles were highly dispersed, uncovered, and
protected from agglomeration by C3N4 which is critical for their function and catalytic
applications. Both Pd(II) and Pd(0) catalysts show very good catalytic activity for Sonogashira
crossꢀcoupling reactions involving aryl iodide and aryl alkynes under copperꢀfree and phosphineꢀ
free conditions and are stable for repeated reuses.
2. Experimental
2.1.
Materials.
Cyanamide,
poly(ethylene
glycol)−block−poly(propylene
glycol)−block−poly(ethylene glycol, average molecular weight 5800) (PEG−PPG−PEG),
tetraethyl orthosilicate (TEOS), palladium acetate [Pd(OAc)2], and sodium borohydride (NaBH4)
were obtained from Sigma Aldrich. All other organic chemicals used under the investigation
were of analytical grade purchased from Avrao and Merckꢀchemicals, unless otherwise
specified.
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2.2. Synthesis of mesoporous silica matrix SBA-15. The template SBAꢀ15 for replication of
carbon nitrides was synthesized according to our previous report.37 In a typical procedure
following molar gel composition was used to obtain SBAꢀ15 silica matrix: SiO2 : 0.012
(PEG−PPG−PEG), 6.5 HCl : 169 H2O. The resultant gel was stirred for 20 h at 313 K and then
aged at 353 K for 48 h. The product after crystallization was washed thoroughly with deionized
water, dried at room temperature. The surfactants were removed by calcinations in a muffle
furnace at 773 K for 6 h.
2.3. Synthesis of mesoporous carbon nitrides (g-C3N4). The lowꢀmelting solid cyanamide was
heated at 323 K which allows effective filling of silica matrices by soaking. The SBAꢀ15 silica
materials was mixed with liquid cyanamide at 343 K and stirred for 30 min. The
silica/cyanamide composites were then taken out, dried in air, and the surfaceꢀadsorbed
cyanamide molecules were removed by washing with ethanol. The dried composite material was
heated under flowing N2 at 823 K for 4 h. The calcined materials were then crushed into a fine
powder and treated with hydrofluoric acid (5 %) twice for 24 h to free mesoporous silica matrix
and obtain mesoporous carbon nitrides (g−C3N4). The centrifuged carbon nitride powder was
separated from the HF solution and washed with distilled water.
2.4. Synthesis of palladium-grafted mesoporous g-C3N4. Two different Pdꢀcatalysts with
different oxidation states were prepared. For Pd(II)ꢀcatatlyst, mesoporous graphitic carbon
nitrides was suspended in water followed by addition of Pd(OAc)2 and the suspension was
subjected to sonicated for period of 10 min. Then the suspension was further magnetically stirred
at room temperature for a period of 24 h. The Pd(II)ꢀanchored C3N4 was collected by filtration,
washed thoroughly with ethanol and was slowly dried at room temperature for 24 h to yield final
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material which is denoted as Pd(II)ꢀC3N4 [(Pd(II)ꢀMCN] for convenience. The catalysts were
further dried under vacuum at 423 K for 8 h and stored in a vacuum decicator. In the second
procedure to prepare Pd(0)ꢀcatalysts, to the suspension of C3N4 and Pd(OAc)2 obtained by
sonication, sodium borohydride solution (NaBH4) was slowly added under nitrogen atmospheres
and stirred for a period of 24 h at room temperature. The powder samples were collected by
centrifugation, washed thoroughly with Millipore water, and ethanol to obtain Pd(0)
nanoparticles stabilized by conjugated πꢀnetwork of graphitic C3N4. Finally the sample was
further dried at 423 K under vacuum for 8 h to remove traces of moiture and the resulting
material is denoted as Pd(0)ꢀC3N4 [(Pd(0)ꢀMCN].
2.5. Characterization of materials. Lowꢀangle and wideꢀangle powder Xꢀray diffraction
patterns were recorded using a MAC Science powder diffractometer with CuKα radiation (40
kV, 200 mA) with 0.02° step size over desired 2θ range. The textural properties such as BET
surface area, mesoporeꢀsize distribution, and total pore volume of Pd(II)ꢀC3N4 and Pd(0)ꢀC3N4
were evaluated using nitrogen adsoption/desorption isotherms with BELSORB Mini instrument.
The morphology of the samples was confirmed from TEM (JEOLꢀ2000EX II, 200kV). The
carbonꢀ and nitrogenꢀcontents were determined by CHN analysis (PerkinElmer 4100). Elemental
analysis of Pd was performed with a Perkin Elmer Plasma 400 ICPꢀOES. The 13C crossꢀ
polarisation magic angle spinning (CPꢀMAS) NMR spectrum was obtained with a Bruker
Advance 300 spectrometer. The thermal analyzes (TG/DTA) were acquired with a Shimadzu
1
13
Thermal Analyzer. H NMR and C NMR spectral data were recorded with a Bruker 500 MHz
spectrometer. In order to confirm the presence of Pd, C, N, and O in the solid material, Xꢀray
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photoelectron spectroscopy (XPS) technique has been performed using an Omicron
Nanotechnology XPS serial no: 0571 spectrometer using Mg Xꢀray source.
2.6. General procedure for sonogashira coupling reactions.
To amixture of aryl iodide (3 mmol) and arylalkyne (3.6 mmol) in DMF (4.5 mL) at 353 K was
added pyrolidine (3 mmol) and stirred for 4 h. To this solution was added 5 wt % catalyst (with
respect to starting material). The progress of the reaction was monitored by the gas
chromatography. After the reaction was complete, reaction mixture was poured into water (10
mL) and stirred for 10 min at room temperature. Product was extracted using ethyl acetate (3 x 5
mL). Combined organic layer was dried over anhydrous Na2SO4, filtered, and concentrated
under reduced pressure to obtain a crude compound. The crude compound thus obtained was
purified by flash chromatography on silica gel using ethyl acetate:hexane (1 to 10% ) as eluent to
1
get the pure compound. The isolated products were confirmed by HꢀNMR, and 13CꢀNMR
spectral data.
3. Results and discussion
Figure 1 shows the smallꢀangle powder XRD patterns of Pd(II)ꢀMCN obtained by replication of
SBA−15 as hard template and exhibits three wellꢀresolved diffraction peaks with prominent peak
at 0.9 degree and two weak peaks at 1.61 and 1.84 2θ, which could be indexed as the 100, 110,
and 200 diffractions similar to parent siliceous SBA−15 of p6m symmetry38 (Figure S1
Supporting Information). The intensity of diffraction peaks of MCN suggests effective
replication and high structural order of mesoporous carbon nitrides. Thermal analysis (TG/DTA)
showed no significant amount of silica content in HF treated MCN after combustion of the MCN
more than 1273 K in the presence of air (Figure S2, Supporting Information). To assess the
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nature of mesoporous wallꢀstructures whether it is amorphous or crystalline, the MCN was
characterized by wideꢀangle XRD. The wide angle XRD (Figure 1, inset) of MCN is
characterized by a very strong reflection centered at 3.21 Å and this peak is reminiscent of the
002 basal plane reflection in graphite that occurs at 3.32 Å. Thus wideꢀangle XRD pattern
suggests a graphiteꢀlike structure, showing the typical (002) interlayerꢀstacking peak,
corresponding to an interlayer distance of d = 0.321 nm, in agreement with literature report.39
To gain information on the local coordination environment in C3N4 materials, solidꢀstate
13C MAS NMR was performed and the spectrum (Figure 2) exhibits one broad peak centered at
147 ppm along with two other peaks at 142 and 134 ppm, consistent with sp2ꢀbonded carbon
(Figure 2) from sꢀtrizazine rings.40 The FTIR spectrum of C3N4 shows (Figure S3 supporting
Information) broad bands of the stretching and deformation vibration modes of NH2 groups at
3418 and 1621 cm−1 (as well as overlapping band of OH stretching), along with weak band at
2162 cm−1 (not marked in the spectrum). The bands at 1462, 1417, 1329 and 805 cm−1 are
characteristics for sꢀtrizine ring vibrations. The strong band observed at 1248 cm−1 indicates the
presence of C−N stretch in the 3ꢀfold Nꢀbridge linking the triazine rings. The high resolution
SEM images (Figure 3) of samples Pd(0)ꢀMCN and Pd(II)ꢀMCN exhibit rodꢀlike morphologies
similar to previous reports41 indicating a layered structure of graphitic C3N4.
Highꢀresolution TEM was used to investigate the structural ordering and morphology of
mesoporous C3N4 materials. Figure 4A shows the TEM images of Pd(II)ꢀMCN where bright
contrast strips on the image represent the poreꢀwall images whereas dark contrast cores display
an array of empty mesoporous channels when viewed in the direction perpendicular to their axis.
Furthermore, some of particles also clearly reveal a hexagonallyꢀordered honeycombꢀlike
structure with uniform mesoporous channels in accordance with smallꢀangle XRD pattern.41
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Additionally, the existence of Pd(II) species is also revealed from Figure 4 (inset) as well as
lattices fringes of palladium atom in HRTEM (Figure 4B) and EDX spectra (Figure 4C). Similar
morphological features were also clearly seen over Pd(0)ꢀMCN (Figure 5A) where the materials
was reduced using sodium borohydrides and HRTEM image viewed down the pore axis reveal a
hexagonal mesoporous channels. The Palladium nanoparticles were evenly distributed
throughout the solid and the spherical nanoparticles size were about 2ꢀ3 nm (Figure 5B) and
electron diffraction pattern clearly confirms the presence of palladium from the observed 111,
200, and 220 diffractions (Figure 5B, inset) of palladium nanoparticles.
Nitrogen adsorption/desorption analyses were also used to investigate the textural
properties of mesoporous C3N4 materials. The sample Pd(II)ꢀMCN shows (Figure 6) a sharp rise
in the isotherm at a relative pressure higher than 0.65 indicating the presence of mesopores in the
samples corroborating the TEM and XRD data discussed above. The nitrogen sorption isotherms
are found to be of Type IV in nature as per the IUPAC classification and exhibited a H1
hysteresis loop, which is typical of mesoporous solids consisting of very large mesopores.39 The
BET surface and pore volumes were found to be 355.9 m2/g and 0.635 mL/g, respectively.
Similarly, the sample Pd(0)ꢀMCN (Figure 7) also characteristics of mesoporous solids having
329 m2/g and 0.693 mL/g under similar experimental conditions. The carbon to nitrogen ratio of
samples Pd(II)ꢀMCN and Pd(0)ꢀMCN determined by CHN analysis were found to be 0.69 and
0,64 (theoretical ratio is 0.75), respectively. The Pd metal content estimated by ICP analysis was
found to be 2.65 and 1.46 wt% for samples Pd(0)ꢀMCN and Pd(II)ꢀMCN, respectively.
The XPS spectrum of Pd(II)ꢀMCN in full range is given in Figure 8d confirms the
presence of palladium, carbon, nitrogen, and oxygen in the material without any other impure
elements. The narrow XPS spectrum of palladium exhibits two peaks with binding energy of
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334.8 eV and 343.1 eV corresponding to Pd3d5/2 and Pd3d3/2, respectively. As seen from the
figure it is noticed that the binding energy of Pd has been shifted slightly in comparison to the
metallic Pd.42 In figure 8a & 8b, highꢀresolution XPS spectrum of Pd 3d and 3P regions are
shown, which revealed the presence of Pd with binding energy of 3d3/2 = 343.1 eV, which are
in good agreement with expected binding energy values for Pd(II).43,44 On the other hand
minor peak at the binding energies of about 334.8 and 341.1 eV suggested the presence of low
amount of Pd(0) nanoparticles. The C1s spectrum spectrum of Pd(II)ꢀMCN (Figure 8c) is deꢀ
convoluted into three separated bands i.e. 283.8 eV (sp2), 285.7 eV (C–O) and another band at
287.7 eV (C=O) which confirms the presence of acetate ligands in the solid Pd(II)ꢀMCN
material.45 Thus characterized Pdꢀanchored mesoporous carbon nitride materials with surfaceꢀ
exposed palladium (Scheme 1) were investigated for Sonogashira reaction under heterogeneous
condition.
Heterogeneous supported palladium species are generally required the use of CuI as coꢀ
catalyst in order to realize reasonable activities.46 Therefore, the coupling reaction of
halobenzene with phenylacetylene appears to be of interest for developing new heterogeneous,
copperꢀfree and phosphineꢀfree palladium catalysts. Hence catalytic activity of heterogeneous
Pd(0)ꢀMCN and Pd(II)ꢀMCN (Scheme 1) was investigated for Sonogashira crossꢀcoupling
reaction (Scheme 2). It is worth to mention that Pd(0) nanoparticles are exclusive stabilized by
amine functionalities as well as nitrogen atoms present in extended πꢀconjugated three
dimensional network;26 Whereas Pd(II) species preferentially form coordination complexes with
nitrogen atoms of mesoporous carbon nitrides as evidenced from XPS analyses. For investigation
of catalytic properties, the reaction of iodobenzene (1a) with phenyl acetylene (2a, scheme 2)
was chosen as the model reaction (Table 1). Since heterogeneous catalytic reactions often depend
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on various reaction parameters, the reaction was scrutinized to obtain a maximum yield with
good product selectivity. To optimize the reaction, a series of catalytic experiments were
performed by changing reaction parameters such as solvent, base, amount of catalysts used, and
reaction temperature. We first studied the effect of various solvents while keeping the other
parameters constant. Table 1 refers to results obtained from the coupling reaction for the
preparation of diphenylacetylene (Scheme 2) under identical reaction conditions: iodobenzene (3
mmol), phenylacetylene (3.6 mmol), base (6 mmol), solvent (4.5 mL), Catalyst Pd(II) – MCN (5
wt %) at 353 K for 4 h.
It is seen from Table 1 that Sonogashira reaction was found to be working well with
highest conversion of iodobenzene in DMF (95.9 %, table 1) whereas other solvents like
acetonitrile, ethanol, mꢀxylene, THF, and water lead to lower conversions (2.4 to 56.5 %, Table
1
1, Entries 1ꢀ6). The formation of crossꢀcoupled product was confirmed by H NMR and 13C
NMR spectroscopy (Supporting Information, Fig. S4 and S5)). Another crucial factor that
influences Sonogashira reaction is the nature of base used to abstract protons. Among the two
organic bases tested, pyrrolidine exhibited very good conversion (Table 1, Entry 1). However,
the addition of inorganic bases such as KOH, K3PO4, K2CO3, and Na2CO3 resulted in poor to
moderate conversions (Table 1, Entries 8ꢀ11). Results obtained from the above catalytic runs
suggest that organic amines are more efficient than inorganic counterparts due to the effective
capture of hydrogen from iodobenzene during the coupling reaction. Hence, pyrrolidine was
recognized as the suitable base to couple aryl iodides with acetylenes efficiently (Table 1, Entry
1). Furthermore, as heterogeneous catalysts usually require higher activation temperature than
the homogeneous counterparts, the influence of temperature over coupling reaction was also
performed. For this purpose, reaction was performed at three different temperatures of 333, 353
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and 373 K by using above optimized conditions (DMF as solvent and pyrrolidine as base). From
Table 1 it is noticed that no appreciable reaction occurs at 333 K (Entries 17ꢀ19) and increasing
the reaction temperature to 353 K lead to the excellent conversions of iodobenzene with good
yield for desired product (Table 1, Entries 12,13). However, there was no significant reduction in
reaction duration (~ 3h) or conversion level when the reaction temperature was raised to 373 K
and therefore, 353 K was chosen for subsequent investigations. In addition, optimization of
catalyst loading at different temperatures (Table 1, Entries 12ꢀ19) suggested an optimum catalyst
loading of 5 wt%. Although, slight increase in conversion was observed for catalyst loading of
7.5 % (Table 1, Entries 13, 16, and 20), it is insignificant when compared to catalyst versus
product ratio. As expected, decrease in the conversion of the product (48.3 %, Table 1, Entry 12)
was observed when the catalyst loading was reduced to 2.5 wt %. This incomplete conversion
might be due to an inadequate number of catalytically active species (Pd). The above reaction
conditions were adopted to extend the scope of the Sonogashira reaction of various aryl iodides.
The optimized reaction conditions were used to explore Sonogashira crossꢀcoupling
reactions between various aryl iodides and phenyl acetylenes and the results are summarized in
Table 2. In general, the reaction proceeded very well with a wide range of substrates containing
both electron donating and electron withdrawing substituents on iodobenzenes to afford good to
excellent yields using 5 wt% of Pd(0)ꢀMCN catalysts (Table 2, entries 1–10). For instance
electronꢀdonating groups such as methoxy, methyl, hydroxyl and −NH2 groups provide almost
quantitative conversion as well as selectivity for the desired products (Table 2, Entries 4ꢀ7). But
there was slight decrease in the yield for hydroxyl and amine substituted compounds (Table 1,
Entries 4 and 7); however it is important to mention that a small amount starting materials left
unreacted and no side product is formed. Similarly, all the aryl iodides with electron withdrawing
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group afforded the crossꢀcoupled products with nearly complete conversion with excellent
selectivities under optimized reaction conditions (Table 2, Entries 2, 3, 8ꢀ10). The catalytic
activity per catalytic sites, that is the turnꢀoverꢀnumber (TON, number of moles of products
formed per moles of Pd) of this catalysts were ranges from 142.5 (entry 1) to maximum of 150
(entry 2) as seen from Table 2. The high activities of the catalysts can be attributed to uniformly
distributed Pd nanoparticles stabilized by πꢀconjugated graphitic carbon nitrides which show
synergetic effect towards high catalytic performance. Furthermore, the investigation of Pd(II)ꢀ
MCN exhibits still better catalytic activity (Table 3) compared to Pd(0)ꢀMCN under identical
experimental conditions. Pd(II) catalysts also exhibits very high catalytic activity for coupling
between phenyl acetylene and various aryl iodides containing both electron withdrawing and
electron donating groups. More interestingly, the observed TON ranges from 208 to 230.7 for
various substrates which is considerably higher than the Pd(0)ꢀMCN catalysts. The high catalytic
activity of these catalysts may be attributed to the readily accessible surface exposed Pdꢀ
nanoparticles which inherently favours the C−C bond formation reactions.
For investigation of Pd leaching from the solid Pd(0)ꢀMCN catalyst to the solution phase
or whether the catalyst is truly heterogeneous in nature, hot filtration test was carried out. The
coupling reaction of phenylacetylene and iodobenzene under optimized conditions was
performed initially in presence of catalysts for a period of 1 h, the catalyst was quickly separated
by filtration and the filtrate was then analyzed by GC to check the conversion which was found
to be 28.4 %. The filtrate was then continued to reflux for another 3 h under the same conditions
but without any catalyst. GC analysis revealed no increase in the conversion values (28.6 %) in
the absence of catalyst suggesting the catalyst is truly heterogeneous in nature because no
leaching of palladium from mesoporous carbon nitrides was noticed as confirmed from ICP
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analysis. Thus the mesoporous graphitic carbon nitride containing palladium could serve as an
efficient heterogeneous catalyst for C−C cross coupling reactions. One of the main criteria for
using heterogeneous catalysts from an industrial perspective is the Longꢀterm stability, recovery
and reusability of the catalyst. To investigate reusability, we performed the cross coupling
reaction under optimized condition for 4 h as previously described and then the catalyst was
filtered, washed thoroughly with ethanol several times to remove the base as well as other
organic compounds. The separated catalyst was dried in vacuum under ambient conditions and
the catalyst was reused in another six successive catalytic cycles under identical reaction
conditions and results show very good conversion with negligible decrease in catalytic activity
(Figure 9). Furthermore, the morphological and textural features of Pdꢀcatalysts after first and
fourth reuses were also evaluated using TEM observation (Supplementary Information, Fig. S6
& S7). The TEM images reveal that Pdꢀparticles reasonably retain their spherical morphology
without any appreciable aggregation. Thus the catalyst can be reused without any appreciable
change in the conversion and selectivity of crossꢀcoupled products.
All the experimental findings of catalytic data and absence of any cross coupling reaction
without any catalysts suggest the catalytic role played by Pdꢀgrafted mesoporous graphic carbon
nitrides in this reaction. In accordance with previous literature report,47 a plausible reaction
pathway is proposed (Scheme 3). Initially the oxidative addition of aryl iodide to Pd(0) leads to
formation of Pdꢀcomplex (A) followed by insertion of alkyne to form πꢀbond mediated new Pdꢀ
complex (B). Deportonation of alkyne groups of Pdꢀcomplex B leads to formation of anionic Pdꢀ
complex (C). This complex undergoes iodide displacement (D) followed by reductive
elimination to give the coupled product (E).
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4. Conclusions
We successfully developed a mesoporous graphitic carbon nitride polymer matrix for the
immobilization of surface exposed Pd nanoparticles by locking them within a mesoporous threeꢀ
dimensional πꢀnetwork. The grafted Pd nanoparticles exhibit a wide substrate scope for the
Sonogashira crossꢀcoupling of various aryl iodides with phenylacetylene. Lowꢀangle XRD,
TEM, and EDX results confirmed very high crystallinity as well as high degree of Pd dispersion
in Pd nanoparticles πꢀconjugated graphitic mesoporous carbon nitrides. Nitrogen sorption data
confirmed mesoporosity with high surface areas. The molecularly defined Pd(II)ꢀMCN catalyst
exhibited higher turnꢀoverꢀnumber (TON) than the corresponding Pd(0)ꢀMCN catalysts under
identical conditions. Hot filtration tests confirmed the heterogeneity of the catalysts and the
catalyst was reused for six successive reactions without any significant change in its catalytic
activity. The ordered mesoporous carbon nitride with high nitrogen content coupled with
excellent textural properties and chemical stability of the resulting catalysts, easy recovery and
regeneration indicate the possible application of fine chemical synthesis of industrial importance.
The present approach is applicable to produce surfaceꢀexposed metal nanoparticles through πꢀ
electron stabilization and synergetic effect of mesoporous carbon nitrides towards activation of
various chemical bonds under heterogeneous catalytic process.
Acknowledgement
M. S. thanks Science & Engineering Research Board for their research support (No SB/S1/PCꢀ
043/2013).
Supporting Informations
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Small angle XRD pattern of SBAꢀ15 silica template, TG/DTA curves and FTIR spectrum
1
13
mesoporous carbon nitrides, H NMR and C NMR of a coupling product and TEM images of
Pd catalysts after first and fourth reuses.
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Figure 1.Smallꢀangle and wideꢀangle Xꢀray diffraction spectra of mesoporous carbon nitrides
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Figure 2.13C MAS NMR spectrum of mesoporous graphitic carbon nitrides
Figure 3.FESEM images Pd(II)ꢀMCN and Pd(0)ꢀMCN mesoporous carbon nitrides
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HRTEM
Pd(II)ꢀMCN
EDX spectrum
Figure 4.TEM images of (A) mesoporousPd(II)ꢀMCN (inset figure shows Pd particles); (B)
HRTEM showing lattice fringes of Pd, and (C) EDS spectrum of Pd(II)ꢀMCN.
Pd(0)ꢀMCN
Figure 5.TEM images of (A) mesoporousPd(0)ꢀMCN and (B) Magnified image showing the Pd
nanoparticles along with electron diffraction patter (inset).
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Figure 6. Nitrogen adsorption/desorption isotherms of mesoporousPd(II)ꢀMCN and inset figure
shows pore size distribution.
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Figure 7. Nitrogen adsorption/desorption isotherms of mesoporousPd(0)ꢀMCN and inset figure
shows pore size distribution.
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Figure 8. XPS spectra of (a) Pd 3d, (b) Pd 3p, (c) C 1s and (d) full range survey spectrum
containing all elements of the Pd(II)MCN catalysts.
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A
B
Figure 9. Regeneration and recyclable test for 6 successive cycles: (A) Pd(0)ꢀMCN and (B)
Pd(II)ꢀMCN.
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Pd (II) is chelated with Nꢀ
atoms of Melamine units.
Pd (0) nanoparticles are surfaceꢀ
stabilized by Melamine units of
mesoporous carbon nitrides.
Scheme 1. Schematic representation of Pd(0)ꢀMCN and Pd(II)ꢀcomplexes supported on
mesoporous carbon nitrides (MCN).
Scheme 2. Sonogashira coupling reaction of aryl iodides with phenyl acetylene catalyzed by
copper free Pd (II) –MCN.
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Table 1. Optimization of various reaction conditions of crossꢀcoupling between iodobenzene and
phenylacetylene using Pd(II)ꢀC3N4
Conversion
Entry Catalyst, wt %
Base
Temp. K
Solvent
[%][a]
95.9
56.5
16.7
2.4
1
2
5.0
5.0
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Triethylamine
KOH
353
353
353
353
353
353
353
353
353
353
353
353
353
373
373
373
333
333
333
353
DMF
Acetonitrile
Ethanol
THF
3
5.0
4
5.0
5
5.0
mꢀxylene
Water
DMF
32.9
24.0
85.0
11.7
3.0
6
5.0
7
5.0
8
5.0
DMF
9
5.0
K3PO4
DMF
10
11
12
13
14
15
16
17
18
19
20
5.0
K2CO3
DMF
7.9
5.0
Na2CO3
DMF
9.0
2.5
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
DMF
48.3
97.5
50.5
96.5
98.3
4.0
7.5
DMF
2.5
DMF
5.0
DMF
7.5
DMF
2.5
DMF
5.0
DMF
8.0
7.5
DMF
11.0
Nil
No catalysts
DMF
Reaction conditions: Iodobenzene (3 mmol), Phenylacetylene (3.6mmol), Base (6 mmol),
solvent (4.5 mL), Reaction time was 4 h. [a] GC analysis, conversion based on iodobenzene.
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Table 2. Sonogashira crossꢀcoupling of various iodobenzene and phenylacetylene using Pd(0)ꢀ
C3N4 (MCN) under optimized reaction conditionsa
S.No
1
R1
R2
Product
Yield [%][a]
95.9
TONb
142.5
I
2
3
4
5
6
7
100
100
97.6
100
100
100
96.5
100
100
150
150
NC
NC
I
COOH
I
COOH
146.4
150
H2N
I
I
H2N
H3C
H3C
150
H3CO
HO
H3CO
HO
I
150
I
F3C
F3C
8
9
144.9
150
I
HOOC
O2N
I
HOOC
O2N
10
150
I
aReaction conditions: Iodobenzene (3 mmol), Phenylacetylene (3.6mmol), Base (6 mmol),
solvent (4.5 mL), Catalyst (5 Wt %), Reaction time was 4 h. [a] GC analysis.
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Table 3. Sonogashira crossꢀcoupling of various iodobenzene and phenylacetylene using Pd(II)ꢀ
C3N4 (MCN) under optimized reaction conditionsa
S.No
1
R1
R2
Product
Yield [%][a]
90.2
TONb
208.3
I
2
3
4
5
6
230.7
230.7
230.7
210.0
230.7
230.7
230.7
230.7
100
100
100
90.9
100
100
100
100
NC
NC
I
COOH
I
COOH
H2N
I
H2N
H3CO
H3CO
HO
I
I
HO
F3C
F3C
7
8
I
HOOC
O2N
I
HOOC
O2N
9
I
aReaction conditions: Iodobenzene (3 mmol), Phenylacetylene (3.6mmol), Base (6 mmol),
solvent (4.5 mL), Catalyst (5 Wt %), Reaction time was 4 h. [a] GC analysis.
29